Spray-drying is a typical method of manufacturing powder laundry detergents and involves combining inorganic builder mixtures such as alkali metal bicarbonate, alkali metal carbonate, alkali metal silicate or water-insoluble builders such as zeolite, with water, to form a concentrated slurry. Such slurries typically contain surfactants which are usually anionic in nature, such as linear alkylbenzene sulfonate, alcohol ether sulfates, alcohol sulfates, secondary alkane sulfonates, alphaolefin sulfonates etc. Nonionic surfactants, although not normally included in the crutcher, can be incorporated in the crutcher in small amounts; however, particular attention needs to be devoted to environmental concerns related to "pluming" associated with the spray drying of such slurries. A crutcher composition typically constitutes about 45%-60% solids although it is possible to have a solids content greater than 60% in the crutcher.
Powder detergent compositions typically involve the addition of substantial amounts of alkali metal carbonates, such as sodium carbonate, to the crutcher mix. Alkali metal carbonates, in particular sodium carbonate, can constitute a substantial percentage of the powder detergent formulation, and are added primarily to remove hardness ions such as calcium, via an ion exchange mechanism, and also to provide alkalinity to the wash liquor. In a typical powder detergent manufacturing process, the crutcher mix is processed through a spray tower at very high temperatures to form dry beads. If the detergent formulation contains nonionic surfactants or heat-sensitive ingredients, these additives are sprayed on and absorbed into the dried beads.
A common problem associated with crutcher slurries that contain significant amounts of alkali metal carbonates is their tendency to gel, particularly in the presence of anionic surfactants. This gelling significantly increases the viscosity of the crutcher slurry and makes the crutcher slurry very difficult to process.
In order to reduce the gelation of such slurries for processing, polymeric dispersants have been added to the crutcher mix. Examples of such additives are polycarboxylate polymers such as acrylic polymers and acrylic/maleic copolymers which are added in small amounts, typically about 5% based on the weight of the detergent composition. The addition of polycarboxylates results in the dispersion of solids in the crutcher and thereby reduces the viscosity of the crutcher slurry.
U.S. Pat. No. 4,368,134 teaches the use of watersoluble citric acid salts along with magnesium sulphate to reduce the viscosity of aqueous detergent slurries. U.S. Pat. No. 4,362,640 teaches a method for reducing the viscosity of carbonate based crutcher slurries during the addition of aqueous sodium silicate by adding CO.sub.2 with the silicate solution. U.S. Pat. No. 4,311,606 teaches a method of reducing the viscosity of carbonate based crutcher slurries through the addition of sodium sesquicarbonate along with citric acid. However, the additives listed in the prior art described above function merely as dispersants and the viscosity reduction achieved via these methods is modest.
The inventors have now found that hydrophilic copolymers when incorporated in small amounts in the crutcher slurry composition give a substantial decrease in the viscosity of the slurry compared to the viscosity reducers known in the art. The viscosity decrease with the hydrophilic polymers may be two to three orders of magnitude lower than the viscosity achieved without the polymer in the slurry.